1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a recording sheet by discharging ink droplets from nozzles and a method of detecting a defective nozzle that cannot discharge ink droplets.
2. Description of the Related Art
An ink jet printer, as one type of an image forming apparatus, includes a print head in which a plurality of nozzles are arranged and forms an image on a recording sheet by discharging ink droplets on the surface of the recording sheet from nozzles provided in the print head. There are cases where it is difficult for the ink jet printer to discharge ink droplets due to the clogging of some of the nozzles caused by an increased viscosity of the ink, mixing-in of air bubbles, attachment of dust or paper powders, or the like.
In a case where there is a defective nozzle that cannot discharge ink droplets and the formation of an image is continued in such a state, the image quality markedly deteriorates. Accordingly, it is preferable to promptly detect a defective nozzle and perform a countermeasure such as a cleaning process quickly. In particular, a line-type ink jet printer in which a plurality of print heads are arranged in the main-scanning direction has a high printing speed, and accordingly, considerable damage occurs if a late countermeasure is taken after the occurrence of a defective nozzle.
From this viewpoint, in the field of ink jet printers, various technologies have been proposed for detecting a defective nozzle. For example, as disclosed in Japanese Patent Application Laid-open No. 2009-132025, there is a known technology which employs a light emitting element that emits detection light in a direction intersecting the discharge direction of ink droplets and a light receiving element that receives scattered light generated by irradiating the ink droplets discharged from nozzles with the detection light. According to the technology, a defective nozzle is detected by determining the discharged state of the ink liquid based on the output of the light receiving element.
In addition, as technologies for detecting a defective nozzle during an operation of forming an image on a recording sheet (during a printing operation), there are known exemplary technologies such as the technologies disclosed in Japanese Patent Application Laid-open No. 2007-15193 and Japanese Patent No. 3991279.
According to the technology disclosed in Japanese Patent Application Laid-open No. 2007-15193, a sensor detects an end of a recording sheet is used to determine a space, in which a recording sheet is not present, between two recording sheets which are sequentially conveyed during a printing operation, and a defective nozzle is optically detected by sequentially discharging ink droplets from each nozzle every time the space is encountered.
In addition, according to the technology disclosed in Japanese Patent No. 3991279, a defective nozzle can be optically detected during a printing operation by adjusting the timing to discharge an ink droplet from a nozzle that is a detection target to be earlier or later than the timing to discharge an ink droplet from another nozzle.
However, in a case where a defective nozzle is optically detected during a printing operation, the detection light emitted from a light emitting element is likely to be blocked due to the floating of a conveying belt that conveys a recording sheet or the floating of a recording sheet placed on the conveying belt to make it difficult to appropriately detect a defective nozzle. Particularly in a line-type ink jet printer, during a printing operation, a print head is fixed to a position which faces, with a gap of about 1 mm therebetween, the conveying belt that revolves at a high speed. Accordingly, even when a slight floating of the conveying belt or the recording sheet occurs, the detection light used for detecting a defective nozzle is blocked, and thus it is difficult to perform an appropriate detection.
In the line-type ink jet printer, although various technologies for suppressing the floating of the conveying belt have been proposed, it is difficult to completely eliminate the floating of the conveying belt. Accordingly, the detection light is likely to be temporarily blocked due to the unexpected floating of the conveying belt or the recording sheet. Thus, in a conventional technology, it is difficult to determine the cause of the decrease of the detection light when the light is blocked by the floating of the conveying belt or the floating of a recording sheet. Accordingly, an event in which an output of a light receiving element decreases due to the floating of the conveying belt or the floating of the recording sheet is determined that an ink droplet is not discharged. Thus, there has been a problem in that the accuracy of detection of a defective nozzle decreases.